1. Field of the Invention
The present invention relates generally to wireless communications, but more specifically to methods and systems for cancellation of noise and spurious signal components in a wireless transmitter.
2. Background
A transmitter in a wireless network is responsible for generating a high power output signal with adequate signal strength to deliver a sender""s message. Additionally, several components of the transmitter system work together to ensure that a clean, distortion-free signal reaches the receiver. One such component is the transmitter""s radio frequency (RF) filter system. The filter stage ensures that the integrity of a sender""s message is not threatened by the many compromising system components that the signal encounters as it progresses through the transmitter. Intermodulation distortion (intermod) is perhaps the most well-known example of such a component.
Intermod is a normal byproduct of signal modulation. Intermod is the result of unwanted harmonic byproducts that naturally occur at the output of the mixing stage. Modulation byproducts include odd- and even-order harmonics, some of which remain in the passband of the filter system and corrupt the purity of the message bit stream. Not all harmonics survive modulation with enough power to have a damaging effect on the signal; but, those harmonics that do make it through the modulation stage must be filtered out prior to signal amplification in order to maintain signal integrity through the transmission channel and at the receiver.
A filter system usually rejects the unwanted harmonic byproducts of modulation with ease because the odd- and even-order products occur at predictable frequencies. However, Intermod is not the only source of undesirable signal components. Spurious signal distortion (spurious) is a well-known phenomenon with multiple sources that also contributes to signal degradation. Spurious is especially harmful because it often results in unwanted signal spurs spilling into the bandwidth of the receive channel. Transmitter components such as oscillators, clocks, and automatic gain control devices, to name a few, all may contribute spurious signal components into the mix.
Rejection is both a qualitative and a quantitative measure of a filter system""s capacity to eliminate both noise and spurious components. Efforts to filter spurious from a signal depend in large part upon a filter system""s ability to reject sideband noise and spurious signal components without introducing undesirable filter side effects. One such side effect, for instance, is power loss caused by poor impedance match between filter input or output and upstream or downstream transmitter components. The parasitic capacitive coupling caused by impedance mismatch drains RF energy from the signal.
Methods of eliminating noise and spurious include feedback, feed forward, signal predistortion and the like. However, these methods are not suitable in all situations. For example, in a filter system that relies on passive components, or is otherwise designed to have less than unity gain, contemporary feedback techniques are ineffective because the absence of active components means the output signal is incapable of supplying adequate power to the feedback path. Furthermore, these techniques have been applied almost exclusively to the linearization of active transmitter components such as the power amplifier, ignoring the filter stage.
The disadvantages of the above mentioned methods previously used to handle harmful noise and spurious are overcome by coupling a passive, frequency-dependent phase shift network in parallel with a transmitter""s filter system.
The method and apparatus for spurious and noise cancellation couples a phase shift network to the filter system of a wireless transmitter. A signal splitter splits the signal into a primary prefiltered signal and a secondary prefiltered signal. Each of the primary and secondary prefiltered signals comprises a desirable signal component centered at a first frequency, and an unwanted signal component centered at a second frequency. The primary prefiltered signal is fed to the input of a filter, which filter generates a filtered output signal. The secondary prefiltered signal is fed to the input of a phase shift network
In one embodiment, the phase shift network comprises passive components of resistance, capacitance, and inductance. The resistive components can be a simple voltage divider network that acts as an attenuator to the secondary prefiltered signal, while the resistive and reactive components together provide the necessary attenuation to filter out the desirable signal component, passing the unwanted signal component. The phase shift network inverts the phase of the unwanted signal component by 180 degrees relative to the primary prefiltered signal before combining the signal with the filtered output signal from the filter.
Finally, the method and apparatus for spurious and noise cancellation in a wireless transmitter achieves significant filter sideband rejection without incurring any noticeable passband degradation.
Further features and advantages of this invention as well as the structure of operation of various embodiments are described in detail below with reference to the accompanying drawings.